Vectorial transport of ions and non-electrolytes is the hallmark of transepithelial transport. The present study will determine the molecular changes that occur within the respective cell membranes during the development of epithelia from free individual cells in tissue culture. We have studied LLC-PK1 cells from pig kidney tubules, which develop a confluent polarized epithelium when grown in approrpiate tissue culture medium. In suspension these cells show no polarization of cell membrane activity and are invested in a plasma membrane lacking specific transport properties - e.g., sodium dependent glucose co-transport. When the same suspension of cells is allowed to grow to a confluent layer in culture, the specific transport processes appear. Figure 34 substantially illustrates the reversible changes which occurs in going from cell suspension to confluent epithelial growth. The development of the tight junctions will be followed by the changes in the electrical resistance and dilution and bi-ionic potentials after plating the cells on a permeable support. The development of the apical membrane will be monitored by the changes in the Na ion co-transport of sugar and aminoacids present in monolayers of LLC-PK1 cells. The development of the basal-lateral membrane will be studied following the changes in the Na ion - K ion - ATPase using an enzymatic assay, (3H) ouabain binding and autoradiography or cytochemistry. The role of the protein and glycoprotein synthesis will be studied using labeled precursors or specific inhibitors and gel electrophoresis after isolation of apical and basal-lateral plasma membrane. Using the cell fusion technique the Na ion co-transport system for sugars in LLC-PK1 cells will be incorporated into MDCK cells, another kidney tubule cell line which lacks its own Na ion co-transport of sugars, to study the mechanism of apical membrane degradation.